The present invention relates generally to image-forming devices, and more particularly to a conveyance of a recordable medium in a multicolor image-forming device. The present invention is suitable, for example for an electrophotographic image-forming device such as a multicolor page printer. Paper (or printing paper) is used as exemplary of a recordable medium to be conveyed in the inventive device, but it is to be understood that other media such as an OHP film are not excluded from applicable media, and a range of usable media is not limited to a specific size or thickness of the sheet.
The xe2x80x9cmulticolor page printerxe2x80x9d as used herein, which is also called multicolor tandem printer, typically employs a plurality of transfer sections, and adopts a method employing a plurality of process units arranged in tandem, each having the transfer section. However, the present invention is not restricted to devices using a process-color printing method that represents full-color images by four colors in yellow (Y), magenta (M), cyan (C), and black (B), but also applicable to devices using a spot-color printing method that employs colored materials in a specific color (e.g., gold, silver, or fluorescent colors). The xe2x80x9celectrophotographic image-forming devicexe2x80x9d by which we mean is an image-forming device employing the Carlson process described in U.S. Pat. No. 2,297,691, as typified by a laser printer, and denotes a nonimpact image-forming device that provides recording by depositing a developing agent as a recording material on a recordable medium. The image-forming device according to the present invention is broadly applicable not only to a discrete printer, but also to various apparatuses having a recording function such as a photocopier, a facsimile unit, a computer system, a word processor, and a combination machine thereof.
With the recent development of office automation, the use of electrophotographic image-forming devices for computer""s output devices, facsimile units, photocopiers, etc. has spread steadily. The demand for multicolor printing rather than a single-color printing is expected to grow more and more in future, for example, in the realm of multicolor laser printers and LED printers for computer outputs, or PPC multicolor copiers realized by a combination of a color scanner and an imaging function by a microprocessor, or the like.
The multicolor tandem printer typically includes a register section, a plurality of process units capable of generating a plurality of colors (image-forming units), and a fixer. The process units are arranged in tandem with respect to a sheet conveying direction, for example, in the order of Y, M, C, and K, from upstream to downstream of the sheet conveying direction. Process-color images are formed by a combination of four colors in Y, M, C, and K, and therefore four process units are normally provided in the multicolor tandem printer.
The register section typically includes a pair of rollers disposed between a pick roller that dispenses out sheets of paper from a paper tray or paper cassette, and a yellow process unit. The register section comes in contact with a top edge of the sheet dispensed out from the pick roller, thereby straightens up an orientation of the sheet, and conveys the same to the yellow process unit.
Each process unit generally includes a photoconductive insulator (photosensitive drum), a charger, an exposure section, a development section, and a transfer section. The charger electrifies the photosensitive drum uniformly (e.g., at xe2x88x92700 V ). The exposure section irradiates a laser light or the like on the photosensitive drum, and neutralizes a potential on an irradiated area (to xe2x88x9250 V or so), to form an electrostatic latent image. The development section electrically deposits a developing agent onto the photosensitive drum using, for example, the reversal process, and visualizes the electrostatic latent image. The reversal process is a development method that forms an electric field by a development bias in areas where electric charge is eliminated by exposure to light, and deposits the developing agent having the same polarity as uniformly charged areas on the photosensitive drum by the electric field. The transfer section forms a toner image corresponding to the electrostatic latent image on a sheet of paper, using, for example, a transfer roller.
The multicolor image-forming device in many instances uses a paper conveyance mechanism that electrostatically adsorbs a sheet of paper onto an endless belt (or transfer belt), to convey the sheet at the same speed as the belt moves. The transfer belt is typically provided so as to be sandwiched between the photosensitive drum and the transfer section in each process unit, and to run between the process units (between yellow and black process units in the above example). Since the transfer roller is pressed onto the transfer belt to bring the sheet into contact with the photosensitive drum for a transfer process, the sheet and the photosensitive drum may easily be brought into intimate contact with each other, and thus a high-quality transferred image may be obtained.
The steps of charging, exposure to light, development, and transfer are repeated four times for four colors in four process units, and a four-fold toner layer is transferred onto a recordable medium. In after-processes, the photosensitive drum from which toner has been transferred is neutralized and cleaned, and residual toner is collected, and recycled and/or disposed. Like this, a multicolor image is represented as a combination and overlap of four colors.
The fixer, which is typically comprised of a pair of rollers, fixes the toner layer onto a sheet of paper, and conveys the sheet along a sheet conveying direction. More specifically, the fixer fuses a toner image by applying heat, pressure, or the like, thereby permanently fixes a color image on the recordable medium, and then conveys the sheet in the sheet conveying direction to a next stage.
However, a conventional multicolor tandem printer is disadvantageously susceptible to relative displacement of each color (i.e., colors likely to be out of register), and unlikely to provide high image quality. The color displacement, which results from a variation of sheet conveyance speeds, is produced between the register section and each process unit, and between each process unit and the fixer, but in principle does not occur between four process units.
The reason why no color displacement is caused between four process units is that each process unit usually uses the same components (a drum, a gear and other parts) and the same topology, and therefore the color displacement is negligible if dimensional and/or driving errors and eccentricity of the components in each process unit are negligible, and that the transfer belt serves to keep the sheet conveyance at a constant speed.
Conversely, the reason why the color displacement is caused between the register section and each process unit, and between each process unit and the fixer is that structures (dimensional accuracy of parts such as a roller""s diameter) and driving forces vary from component to component. Even if the numbers of revolutions of driving sources of each component in the register section, the process units, and the fixer are adjusted, a roller""s diameter or the like are altered over time, which causes speeds of each component to vary. In addition, the pattern and time of the speed variation are not constant.
With consideration given to the above, it would be a conceivable idea that the transfer belt might be extended from the register section to the fixer, to maintain a transfer speed from the register section to the fixer. However, such an extension of the transfer belt to the register section would prevent the register section from coming into contact with a top edge of the sheet, and thus from serving to straighten up an orientation of the sheet (i.e., to align the sheet orientation in the sheet conveying direction). Moreover, the extension of the transfer belt to the fixer would damage the transfer belt due to heat generated in the fixer. Therefore, it is technically difficult to extend the transfer belt from the register section to the fixer.
Further, it would also be a conceivable idea that a variation of the sheet conveyance speed might be detected and fed back to control the sheet conveyance, to prevent the color displacement, however, such a method of preventing the color displacement increases the complexity and cost of the device. Therefore, it is technically and cost-effectively difficult to control a variation of the sheet conveyance speeds in each section.
Therefore, it is an exemplified general object of the present invention to provide a novel and useful image-forming device, in which the above conventional disadvantages are eliminated.
To be more specific, it is an exemplified object of the present invention to provide an image-forming device that can form a high-quality multicolor image at relatively low cost.
In order to achieve the above objects, an image-forming device as one exemplified embodiment of the present invention comprises an endless belt that conveys a recordable medium; plural image-forming units that form a toner layer on the recordable medium via the endless belt; and a fixer that fixes the toner layer on the recordable medium, and a first pressing force applicable to the recordable medium by a last image-forming unit at a downstream side along a direction in which the recordable medium is conveyed among the plural image-forming units is greater than a second pressing force applicable to the recordable medium by another image-forming unit adjacent to the last image-forming unit at the downstream side among the plural image-forming units. According to a conventional image-forming device in which plural image-forming units respectively apply the same force to the recordable medium, at a downstream side along a direction in which the recordable medium is conveyed, a variation rate is large between a pressing force applied to the recordable medium by an image-forming unit at a downstream end and another image-forming unit adjacent to the image-forming unit, and a pressing force applied only by the image-forming unit at the downstream end. However, in the inventive image-forming device, the first pressing force applicable to the recordable medium by the endmost downstream image-forming unit is set to be larger than the second force applicable to the recordable medium by another image-forming unit adjacent to the image-forming unit, and thus the above-described variation rate may be reduced. Accordingly, the instant image-forming device may produce a printed output having reduced color displacement derived from the variation rate.
An image-forming device as another exemplified embodiment of the present invention comprises an endless belt that conveys a recordable medium; plural image-forming units that form a toner layer on the recordable medium via the endless belt; and a register section that conveys the recordable medium to the endless belt, and a fourth pressing force applicable to the recordable medium by a first image-forming unit at an upstream side along a direction in which the recordable medium is conveyed among the plural image-forming units is greater than a fifth pressing force applicable to the recordable medium by another image-forming unit adjacent to the last image-forming unit at the upstream side among the plural image-forming units. According to a conventional image-forming device in which plural image-forming units respectively apply the same force to a recordable medium, at an upstream side along a direction in which the recordable medium is conveyed, a variation rate is large between a pressing force applied only by the image-forming unit at the upstream end, and a pressing force applied to the recordable medium by an image-forming unit at an upstream end and another image-forming unit adjacent to the image-forming unit. However, in the inventive image-forming device, the fourth pressing force applicable to the recordable medium by the foremost upstream image-forming unit is set to be larger than the fifth force applicable to the recordable medium by another image-forming unit adjacent to the image-forming unit, and thus the above-described variation rate may be reduced. Accordingly, the instant image-forming device may produce a printed output having reduced color displacement derived from the variation rate.
Other objects and further features of the present invention will become readily apparent from the following description of the embodiments with reference to accompanying drawings.